Research and development on magneto-optical disks have been intensified as being rewritable optical disks, and some of the magneto-optical disks have been already practically used as external memory designed for computers.
In the magneto-optical disk, a magnetic thin film with perpendicular magnetization is used as a recording medium, and a light is used in recording and reproducing. Thus, compared with a floppy disk or a hard disk wherein a magnetic thin film with in-plane magnetization is used, the magneto-optical disk has a larger recording capacity.
In the magneto-optical disk, a still higher recording density can be obtained by making shorter the diameter of the light spot. A shorter diameter of the light spot is obtained, for example, by making shorter the wavelength of the light beam, or by making larger the number of aperture (NA) of the objective lens. Alternatively, a shorter diameter of the light spot may be obtained with the use of a light interrupting plate for interrupting a central portion of the light beam before it is incident on an objective lens.
However, in practice, it is difficult to make an oscillating wavelength of a laser beam from a semiconductor laser (light source) shorter. Thus, this method presents the problem in that a higher recording density of the magneto-optical disk cannot be obtained.
On the other hand, in the method of making larger the N.A., if the optical axis of the optical system is tilted with respect to the magneto-optical disk, the light spot diameter becomes larger than that obtained in the conventional method. Thus, in this method, the problem is presented in that a more precise adjustment in assembling the magneto-optical disk drive and in the permissible level of the warpage of the magneto-optical disk is required compared with the case of the conventional method.
Furthermore, in the method of using the light interrupting plate, the diameter of the central portion of the light spot, which has a high intensity can be made shorter. However, since a side robe is generated, a portion having a relatively high intensity is formed so as to surround the central portion. Thus, interference by reproducing signals from the portion surrounding the central portion into reproducing signals from the central portion occurs, thereby lowering the quality of the reproducing signal.
In order to counteract the above problems, another method has been proposed, wherein a reflected light from the recording medium is converged onto a slit, and only the central portion of the spot of the reflected light is derived. However, in practice, a new problem is presented in that the structure of an optical system in the magneto-optical disk drive becomes complicated.